Magnetic recording media are widely used in audio tapes, video tapes, computer tapes, disks and the like. Magnetic media may use thin metal layers as the recording layers, or may comprise coatings containing magnetic particles as the recording layer. The latter type of recording media employs particulate materials such as ferromagnetic iron oxides, chromium oxides, ferromagnetic alloy powders and the like dispersed in binders and coated on a substrate. In general terms, magnetic recording media generally comprise a magnetic layer coated onto at least one side of a non-magnetic substrate (e.g., a film for magnetic recording tape applications).
In certain designs, the magnetic coating (or “front coating”) is formed as a single layer directly onto a non-magnetic substrate. In an alternative approach, the front coating is a dual-layer construction, including a support layer on the substrate and a thin magnetic layer (or “upper layer”) formed directly on the support or lower layer. With this construction, the lower layer is thicker than the magnetic layer. The support layer is typically non-magnetic and generally comprised of a non-magnetic powder dispersed in a binder. Conversely, the upper layer comprises a magnetic metal particle powder or pigment dispersed in a binder system. The formulation for the magnetic layer is optimized to maximize the performance of the magnetic recording medium in such areas as signal-to-noise ratios, pulsewidth, and the like.
Magnetic tapes may also have a backside coating applied to the opposing side of the non-magnetic substrate in order to improve the durability, electrical conductivity, and tracking characteristics of the media. As with the front coatings, the backside coatings are typically combined with a suitable solvent to create a homogeneous mixture which is then coated onto the substrate, after which the coating is dried, calendered if desired, and then cured. The formulation for the backside coating or layer also comprises pigments and a binder system.
The magnetic recording medium is formed on a non-magnetic substrate. Conventionally used substrate materials include polyesters such as polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and mixtures thereof; polyolefins (e.g., polypropylene); cellulose derivatives; polyamides; and polyimides. However, such substrates do not add to the dimensional stability of the magnetic recording medium. Further, polymeric films have low modulus values, are sensitive to humidity and have modest strength and durability. Changes in the polymeric substrates to improve certain characteristics can mandate changes in the coating thickness as well, which requires additional research and development.
It would be desirable to have a substrate that would maximize the dimensional stability of the magnetic recording medium formed thereon. It would be beneficial for the final magnetic recording medium product to exhibit low hygroscopic expansion, low thermal expansion, and low expansion under applied stress in order to improve the track density and minimize any overwriting of data. It would also be desirable to have a substrate with increased tensile strength and edge durability.
It has now been discovered that a magnetic recording medium which includes a substrate having a front side and a backside, with a particulate/binder magnetic layer formed over the front side of the substrate, wherein the magnetic medium has a cross-web dimensional difference from the magnetic recording head used therewith of less than 900 μm/meter over a temperature range of about 35° C., and over a 70% relative humidity range will provide superior track density.
It has further been discovered that a magnetic recording medium using a metal or glass film substrate will provide additional dimensional stability without requiring changes in the coating thicknesses due to the substrate having a zero hygroscopic expansion.